Voltage dependent resistor

ABSTRACT

A voltage dependent resistor which has a zinc oxide sintered body which itself has voltage dependent properties. The resistor is made by the steps of: (1) providing a formed body of a powder mixture having, as a major part, zinc oxide and additive; (2) coating on the side surfaces of the formed body a paste having as the solid ingredient composition, at least one member selected from the group of (a) more than 50 mole % of silicon dioxide (SiO2), and less than 50 mole % of bismuth oxide (Bi2O3), (b) the same composition as that of said additive, (c) more than 30 mole % of antimony oxide (Sb2O3) and less than 70 mole % of bismuth oxide (Bi2O3), and (d) more than 50 mole % of indium oxide (In2O3) and less than 50 mole % of bismuth oxide (Bi2O3), (3) sintering the coated body; and (4) applying electrodes to opposite surfaces of the sintered body.

United States Patent [191 Matsuoka et a1.

[ VOLTAGE DEPENDENT RESISTOR [76] Inventors: Michio Matsuoka, 4-chome,

Korigaoka, Hirakata, Osaka-fu; Gen Itakura, 3-14, Miyuki-Higashi-machi,Neyagawa, Osaka-fu; Atsushi Iga, 14-11, l-chome, Daiwa, Takatsuki,Osaka-fu; Takeshi Masuyama, 1-3, l-chome, Yamate-cho, Takatsuki,Osaka-fu, all of Japan 22 Filed: Oct. 29, 1974 21 Appl. No.: 518,944

Related US. Application Data [62] Division of Ser. No. 428,737, Dec. 27,1973.

[4 1 Sept. 9, 1975 [56] References Cited UNITED STATES PATENTS 3,760,3189/1973 Masuyama 338/20 Primary ExaminerC. L. Albritton Attorney, Agent,or Firm-Wenderoth, Lind & Ponack [5 7] ABSTRACT A voltage dependentresistor which has a zinc oxide sintered body which itself has voltagedependent properties. The resistor is made by the steps of: (1)providing a formed body of a powder mixture having, as a major part,zinc oxide and additive; (2) coating on the side surfaces of the formedbody a paste having as the solid ingredient composition, at least onemember selected from the group of (a) more than 50 mole of silicondioxide (SiO and less than 50 mole 7? of bismuth oxide (Bi O (b) thesame composition as that of said additive, (c) more than 30 mole ofantimony oxide (Sb O and less than 70 mole of bismuth oxide (Bi O and(d) more than 50 mole of indium oxide (In O,,) and less than 50 mole ofbismuth oxide (Bi O (3) sintering the coated body; and (4) applyingelectrodes to opposite surfaces of the sintered body.

2 Claims, 1 Drawing Figure [30] Foreign Application Priority Data Dec.29, 1972 Japan 48-3619 Apr. 13, 1973 Japan 48-42488 June 15, 1973 Japan48-68066 Dec. 29, 1972 Japan 48-3610 [52] U.S. Cl. 338/21; 252/518;252/521; 338/20 [51] Int. Cl ..H01c 7/10 [58] Field of Search 338/13,20, 21; 252/518, 252/521; 29/182, 182.5, 610

1 VOLTAGE DEPENDENT RESISTOR This application is a division ofapplication Ser. No. 428,737, filed Dec. 27, 1973. r

This invention relates to the preparation of a voltage dependentresistor the properties of which are due to the bulk thereof, and moreparticularly to a varistor comprising a zinc oxide sintered body havinga high resistance layer of a composition such as silicon dioxide,antimony oxide or indium oxide on the side surface of the sintered body.

Various voltage dependent resistors such as silicon carbide varistors,selenium rectifiers and germanium or silicon p-n junction diodes havebeen widely used for stabilization of voltage or current of electricalcircuits. The electrical characteristics of such a voltage dependentresistor are expressed by the relation:

where V is the voltage across the resistor, I is the current flowingthrough the resistor, C is a constant corresponding to the voltage at agiven current and exponent n is a numerical value greater than 1. Thevalue of n is calculated by the following equation:

g", 2/ 1) gm 2/ where V and V are voltages at a given currents I and 1respectively. The desired value of C depends upon the kind ofapplication to which the resistor is to be put. It is ordinarilydesirable that the value of n be as large as possible since thisexponent determines the degree to which the resistors depart from ohmiccharacteristics.

There have been known voltage dependent resistors comprising sinteredbodies of zinc oxide with or without additives and having silver paintelectrodes applied thereto, as disclosed in the U8. Pat. No. 3,496,512.The non-linearity of such voltage dependent resistors is attributed tothe interface between the sintered body of zinc oxide with or withoutadditives and the silver paint electrode and is controlled mainly bychanging the composition of said sintered body and said silver paintelectrode. Therefore, it is not easy to control the C- value over a widerange after the sintered body is prepared. Similarly, in the voltagedependent resistors comprising germanium or silicon p-n junction diodesit is difficult to control the C-value over a wide range because thenon-linearity of these voltage dependent resistors is not attributed tothe bulk thereof but to the p-n junction. On the other hand, siliconcarbide varistors have non-linearity due to the contacts amongindividual grains of silicon carbide bonded together by a ceramicbinding material, i.e., to the bulk and are controlled with respect tothe C-value by changing the dimension in the direction in which thecurrent flows through the varistors. The silicon carbide varistors,however, have a relatively low n-value ranging from 3 to 6 and areprepared by firing in a non-oxidizing atmo sphere, especially for thepurpose of obtaining a lower C-value. In US. Pat. Nos. 3,663,458,3,669,058, 3,637,529, 3,632,528, 3,634,337 and 3,598,763, there havebeen disclosed voltage dependent resistors comprising sintered bodies ofzincoxide with additives such as bismuth oxide, uranium oxide, strontiumoxide, lead oxide, barium oxide, cobalt-oxide and manganese oxide. Thenon-linearity. of such voltage dependent resistors is attributable tothe bulk thereof and is independent of the interface between thesintered bodies and the electrodes. Therefore, it is easy to control theC- value over a wide range by changing the thickness of the sinteredbody itself. Such voltage dependent resistors of the bulk type have moreexcellent properties with respect to the n-value, transient powerdissipation and AC power dissipation than do SiC varistors.

A disadvantage of the zinc oxide voltagedependent resistors is theirpoor stability in an electric load life test in a high ambient humidity.When DC. power is applied to the zinc oxide sintered body in a highambient humidity, the sintered body shows a decrease in the surfaceelectrical resistance. This decrease causes in particular an increase inthe leakage current in the zinc oxide vo1tage-dependent resistor of thebulk type and results in a poor non-linear property. The deteriorationof the non-linear property of the voltage-dependent resistor occurs evenat a load of low power such as a load lower than 0.01 watt in a highambient humidity, for example 90 R.H. at C. Therefore, it is necessarythat the sintered body is completely protected against outside moistureby a protective coating.

Another disadvantage of the zinc oxide voltage dependent resistorsaforesaid exists in their poor ability to withstand impulse current.When an impulse wave is applied to the zinc oxide sintered body, thesintered body suffers a flashover along its side surface at an impulsevoltage above SOOV/mm, and despite no deterioration in the interior ofsintered body the side surface of the sintered body is heavily damaged.The ability to withstand impulse current is unfavorable particularly forapplication of the varistor as a lightning arrester.

There is other prior art that relates to a voltage dependent resistorcomprising a sintered body comprising a zinc oxide and other additivesand being characterized by a high C-value, high n-value, high stabilitywith respect to temperature, humidity and electric load, and goodability to withstand impulse current. Such a resistor is disclosed inUS. Pat. No. 3,760,318. More specifically, a zinc oxide sintered bodyaccording to said US. Pat. No. 3,760,318 has Li ions or Na ions diffusedinto said sintered body from the side surface thereof at a temperatureof 600C to 1,000C. This diffusing process inevitably results in loweringthe n-value of the resultant resistor in the current region isundesirable for an application requiring low leakage current.

An object of the present invention is to provide a method for making avoltage dependent resistor char acterized by a high stability withrepect to a do load in high humidity and a good ability to withstandimpulse current.

Another object of the present invention is to provide a method formaking a voltage dependent resistor characterized by a high n-value evenin a low current region and a high stability with respect to a dc. loadin high humidity and a good ability to withstand impulse current.

These and other objects of the invention will become apparent uponconsideration of the following description taken together with theaccompanying drawing in which the single FIGURE is a partlycross-sectional vies of a voltage-dependent resistor in accordance withthe invention.

Before proceeding with a detailed description of the manufacturingprocess for the voltage-dependent resistor contemplated by theinvention, the construction of the resultant resistor will be describedwith reference to the aforesaid FIGURE wherein reference character 10designates, as a whole, a voltage-dependent resistor comprising, as itsactive element, a sintered body having surfaces consisting of a sidesurface 2 and opposite end surfaces 3 and 4 to which a pair ofelectrodes 5 and 6' are applied. Said sintered body 1 is prepared in amanner hereinafter set forth and has a high resistance layer 11 at saidside surface 2 and can have any crosssectional form such as circular,square or rectangular.

The process for making 'a voltage dependent resistor of a bulk'typecharacterized by a high humidity resistance and a good ability towithstand current surges according to the invention comprises: (1)providing a formed body of a powder mixture comprising, as a major part,zinc oxide and an additive including Bi O (2) coating on the sidesurfaces of said body a paste comprising, as the solid ingredientcomposition, at least one member selected from the group consisting of(a) more than 50 mole of silicon dioxide (SiO and less than 50 mole ofbismuth oxide (Bizog), (b) the same composition as that of saidadditive, (c) more than 30 mole of antimony oxide (Sb O and less than 70mole of bismuth oxide (Bi O and (d) more than 50 mole of indium oxide111,0 (3) sintering said coated body; and (4) applying two electrodes tothe oppositeend surfaces of said sintered body.

Said zinc oxide sintered body which itselfhas voltage dependentproperties can be prepared by using a composition described in US. Pat.Nos. 3,663,458, 3,669,058, 3,636,529, 3,632,528, 3,634,337 and3,598,763. Among various compositions, greater result can be obtainedwith a composition consisting essentially of, as a major part, 80.0 to99.9 mole% of zinc oxide and, as an additive, 0.05 to 10.0 mole intotal, of at least one member selected from the group consisting ofcobalt oxide (C), manganese oxide (MnO), antimony oxide (Sb Obariumoxide (BaO), strontium oxide (SrO) and lead oxide (PbO).

According to the present invention, the resultant resistor has anexcellent ability to withstand current surges in an irnpulse'currenttest, when said .coating paste comprises, as the solid ingredientcomposition, 70 to 95 mole of silicon dioxide (SiO and 30 to mole ofbismuth oxide (Bi O Similarly, the ability to withstand surge currentcan be improved greatly by using coating paste comprising, as the solidingredient composition, 70 to 95v mole of antimony oxide (Sb O Accordingto the present invention, the ability towithstand surge current can befurther improved by using coating paste comprising, as the solidingredient composition, 50 to 95 mole of silicon dioxide (SiO 2 to 45mole of antimony oxide (Sb O and 2 to 20 mole of bismuth oxide (Bi O Ithas been discovered according to the invention that the DC. stability inhigh humidity and the ability I consisting of cobalt oxide (CoO),manganese oxide (MnO) antimony oxide (Sb O barium oxide (BaO), strontiumoxide (SrO) and lead oxide (PbO);

The sintered body 1 can be prepared by a per se well known ceramictechnique. The starting materials comprising Zinc oxide powder andadditives such as bismuth oxide, cobalt oxide, manganese oxide, antimonyoxide, barium oxide, strontium oxide, lead oxide, uranium oxide and tinoxide are mixed in a wet mill so as to produce a homogeneous mixture.The mixtures are dried and pressed in a mold into desired shapes at apressure from 100 kg/cm to 1,000 mg/cm When a rod-shaped resistor isdesired, the mixed slurry can be fabricated into the desired shape byextruding and then dried. The mixtures may be preliminarily calcined ata temperature of 700 to 1,000C and pulverized for easy fabrication inthe subsequent pressing step. The mixtures may be admixed with asuitable binder such as water, polyvinyl alcohol, etc.

After the mixtures are formed into the desired shapes, the formed bodiesare coated, on the side surfaces, with a paste including powder havingthe same composition as said additive, or a combination of bismuth oxidewithsilicon dioxide, antimony oxide or indium oxide, so as to form ahigh resistence layer at the side surfaces after sintering. Said pastecomprises, as the solid ingredient composition at least one memberselected from the group consisting of (a) more than 50 mole of silicondioxide (SiO and less than 50 mole of bismuth oxide (Bi O (b) the samecomposition as that of said additive, (c) more than 30 mole% of antimonyoxide (Sb O and less than '70 mole of bismuth oxide (Bi O and (d) morethan 50 mole of indium oxide (111203) and less than 50 mole of bismuthoxide (Bi O and as a binding material, an organic resin such as epoxy,vinyl or phenolresin in an organic solvent such as butyl acetate,toluene or the like. Said silicon dioxide, bismuth oxide, antimony oxideand indium oxide can be replaced, respectively, with any siliconcompound, bismuth compound, antimony compound and indium compound suchas an oxalate, carbonate, nitrate, sulfate, iodide, fluoride orhydroxide which is converted into the corresponding oxide at thesintering temperature.

After being coated with said paste, the formed bodies are sintered inair at a temperature of 100 to 1,450C for 1 to 5 hours, and then furnacecooled to room temperature. The sintering temperature is determinedbased on the desired electrical resistivity, nonlinearity stability andthe thickness of the high resistance layer formed at the side surface ofthe sintered body. Also,

the electrical resistivity can be reduced by airquenching from thesintering temperature to room temperature. The sintered body hasnon-ohmic resistance due to the bulk itself. Therefore, its C-value canbe changed without impairing the n-value by changing the distancebetween said opposite end surfaces. A shorter distanceresults in a lowerC-value. The coating paste forms a high resistance layer, as can beproved by measurement of the resistance distribution in the crosssectionof the sintered body, which will show a high resistance atthe sidesurface of the sintered body. The high resistance layer is controlled soas to have a thicknessmore than 10,44. Particularly, it can be shownfrom an x-ray analysis of the cross-sectional portion of sintered body,that 'the'paste comprising a'combination of silicon dioxide'a'nd bismuthoxide or antimony oxide and bismuth oxide forms-a layer having athickness of more'than 3p. and that said layer comprises, in a region toa 3; depth from-the side surface,'mor'e than mole electrolytic platingof Ag, Cu, Ni, Sn etc. vacuum evaporating of Al, Zn, Sn etc. and flamespraying of Cu, Sn, Al, Zn etc. in accordance with the prior well-knowntechniques.

Lead wires can be attached to the electrodes in. a per se conventionalmanner b; using conventional manner by using conventional solder. It isconvenient to employ a conductive adhesive comprising silver powder andresin in an organic solvent in order to connect the lead wires to thesilver electrodes. The n-value of a voltage dependent resistor accordingto this invention does not deteriorate even in a low current region dueto the introduction of the covering layer at the side surface of thesintered body, and ithas a high'stability'with respect to temperatureand humidity and in the load life test, which is carried out at 70C, 90RH at a rating power for 500 hours. The n-value and C-value do notchange appreciably after the load life test. From a surge test, which iscarried out by applying a 4 X 10a sec impulse current twice, it is shownthat this voltage- -dependant resistor has the ability to withstand morethan 2,000A/cm F EXAMPLE 1 Starting materials listed in Table l weremixed in a wet mill for hours. Each mixture was dried and pressed in amold into a disc of 40 mm in diameter and 25 mm in thickness at apressure of 340 kg/cm lThe pressed bodies had the side surface coveredby coating paste including solid ingredients listedin Table l and weredried. Then, the bodies were sintered in air for 5 hours at l,200C andfurnace-cooled. The sintered bodies were lapped to the thickness listedin Table l by a'per se well-known technique. The electriccharacteristics of the resultant resistors are shown in Table 1. It willbe readily understood that the C-value changes in proportion tothe'thickness of the sintered-body.

Size of disc: 32 mm in dia.

Thickness of high resistive layer:

Table 1 Composition Solid Ingredient Thickness C(V) n of Smtered ofSintered Body of Paste Body (at lmA) (0.1lmA) (mol. (mol. (mm) 5 150 15SiO B305 (50) 10 302 14 20 605 15 5 153 15 SiO i 20 605 16 5 155 I 14SiO ZnO (99.0) 10 310 l5 z zi z u V Sb O (90) 10 300 I5 Bi O (10) j 20603 15 5 145 l4 [n 0 (90) I0 300 14 Bi (0.5) (10) 20 600 15 SiO (72) 516 Sb Q, (20) 10 315 16 Bi O 8) 20 615 16 5 510 44 SiO (90) 10 1025; V45 z a I i 20 2040 45 ZnO (97.5) 5 500 45 Y Sb O (90) iai o (0.5 I v 101010 45 C00 (0.5). Bi- O (I0) 20 2010 46 .MnO (0.5) 5 505 45 |n O (90)Sb O .(LO) 10 1010 44 Bi o 20 2015 46 SiO (72) 5' 515 46 Sb O a (20) 101025 46 Bi OQ 8) 20 2040 46 ZnO (99.0) 5 250 22 z a (9Q) Bi O (0.5) 10505 22 Table 1 Continu'ed Composition Solid Ingredient Thickness C(V) nof Sintered of Sintered Body of Paste Body (at lmA) (0.llmA) (mol. (mol.(mm) SiO (90) Bi- O; (0.5) 10 490 8.4

' z u Sb- O 0,5 20 985 8.4 ZnO (99.0) I 200 B1 0 (10 8210 (0.5 20 8 l 5l0 ZnO (99.0) SiO (72) 5 205 ll 81.0. (0.5) 315 0 (20) 10 400 l l SrO(0.5) 131.0. 8) 20 810 l2 EXAMPLE 2 I I 7 carried out by boiling thedisc in pure water for 24 Starting materials of Table 2 were fabricatedinto 2 h0urs.The electric characteristics of the resultant resisvoltagedependent resistors by the same process as that tors areshown in Table2.

of Example 1. An impulse test was carried out by apply- Size of disc: 32mm in dia. and 20 mm in thickness ing a 4 X l0 p.s impulse and theability to withstand cur- Sintering: l,200C for 5 hours rent surge wasthus determined. A humidity test was Thickness of high resistance layer:307J.

Table 2 Composition Solid Ingredient Electric Characteristics ofResultant Resistor of Sintered I of Paste C(V) n lrnpuise Boiling Body 5(mol (at lmA) 0.1-1mA Withstand, Test (mol. (KA) AC SiO 50 131 0., 50605 -5.0 SiO (60) Bi O (40) 605 l5 20 -4.7 SiO (70) Bi O (30) 600 15-4.7 $10. 80 131,0 20 600 16 -3.8 Si0 (90) Bi O (I0) 605 16 -2.9 SiO(95) B1 0 (5) 610 I6 30 -3.2 SiO (l00) Bi O (O) 615 15 30 -3.5 $15.0 30131 0 70) 600 14 20 -5.3 ZnO (99.0)

Sb O;, Bi O (50) 600 14 25 -4.5 81 0., 0.5

Sb O (70) Bi O (30) 600 15 25 -3.5 CoO (0.5)

515,0 (90 131,0 10 603 15 35 -2.7 $15 0 95 191 0,, 5 605 15 30 -3.0$15.0 100 131 0 0 610 14 25 -3.3 ln o (50) Bi O (50) 595 14 20 -5.7111,0 (70) B1 0. (30) 600 I4 25 -4.3 ln O (9.0) Bi o I0) 600 I5 35 -3.l111 0 95 131 0 5 600 15 30 -3.4 ln a (100) Bi O (0) 610 14 30 -3.5 SiO,(50) Bigog (50) I960 42 25 -5.5 SiO Bigog (40) I980 42 30 -4.8 SiO Bi O;30) 2000 44 35 -3.9 SiO, B1 0. (20) 2lOO 44 40 -3.2 Si0 Bi O (I0) 204045 40 l .5 SiO- Bi O (5) 2040 45 35 -2.l SnO (97.5) SiO Bi O (0) 2030 4430 -2.3 Bi O (0.5) Sb Q, (30) 131,0. (70) 1980 44 25 -5. l C00 (0.5) SbO (50) Bi Q, (50) 2000 44 30 -4,9 MnO (0.5) Sb O;, (70) Bi o (30) 200045 35 -3.8 515,0, 1.0 sb o 90 131 0 (10) 2010 46 40 -2.5 Sb O (95 B1 0(5) 2015 45 40 3.1 515 0,, (l00) Bigoa (O) 2020 45 30 -3.5 ln O (50) B10. (50) 1990 44 25 -5.3 In- O; (70) Bi 0;, (30) 2005 44 30 -4.9 ln O(90) B1 0. (l0) 20l5 46 40 -3.l 111 0. (95) B1 0. (5) 2015 45 40 -3.4 InO l00) B00 (0) 2000 45 25 -3.4

50 45 5 600 15 30 -4.4 ZnO (99.0) 5 30 20 600 I5 30 -4.8 Bi o (0.5) 95 32 6 l 5 I6 35 -3.2 C00 (0.5) 95 2 3 615 I6 40 -3.4 58 40 I 2 6l0 15 35-3.0 78 I 2 20 6l0 l5 40 2.5 72 20 8 620 I7 45 -l.7 50 45 S 2050 44 40-3.4 ZnO (97.5) 50 30 20 2065 45 45 -2.8 131 0., (0.5) 95 3 2 2045 45 50-217 C00 (0.5) 95 I 1 2 3 2075 46 50 -2.7 MnO (0.5 58 40 v 2 2060 44 50-2.0 515 0., (1.0) 78 2 20 2080 I 46 55 -1.2

. 9 10 EXAMPLE 3 acteristics of the resultant resistors are shown inTable Starting materials of Table 3 were fabricated into voltagedependent resistors by the same process as that Size of disc: 32 mm india. and 20 mm in thickness of Example I. Then the tests were carriedout by the Sintering: 1,200C for 5 hours same methods as those ofExample 2. The electric char- 5 Thickness of high resistance layer:30,12

Table 3 I Electric Characteristics of Composition of Sintered ResultantResistor Body Solid Ingredient Impulse Boiling (mol 76) of Paste C( V) nwith- Test ZnO Bi O; Further (moi "711) (at lmA) 0.l-lmA stand AC(%)Additives (KA) 99.90 0.05 C60 0.05 350 -6.2 89.95 0.05 C60 10 {$10 90420 12 18 -6.2 89.95 10 C00 0.05 51 0., (10 420 13 -3.9 80.00 10 C60 10750 14 20 -4.0 99.0 0.5 C60 0.5 605 16 35 -6.3 99.90 0.05 MnO 0.05 50013 15 -6.3 89.95 0.05 MnO 10 600 14 15 -5.9 89.95 10 MnO 0.05 {Sb O 90900 18 -3.3 80.00 10 MnO 10 131 0 (10 1250 17 25 -3.5 99.0 0.5 MnO 0.51000 23 2.8 99.90 0.05 815 0., 0.05 300 7.9 15 -7.0 89.95 0.05 815 0 10111 0., 90 800 7.2 15 -5.5 89.95 10 815 0 0.05 720 8.2 17 -3.9

z a 80.00 10 Sb O 10 1300 8.6 18 -4.2 99.0 0.5 58 0 0.5 990 8.4 25 -2.099.90 0.05 B60 0.05 320 7.2 18 -5.3 89.95 0.05 B210 10 $10 90 470 8.0 15-4.9 89.95 10 B210 0.05 510 9.4 20 I -2.9

Z II 80.00 10 BaO 10 1200 0.5 20 -3.4 99.0 0.5 BaO 0.5 815 10 25 -2.599.90 0.05 SrO 0.05 300 9.2 12 -7.2 89.95 0.05 SrO 10 515 0;, 90 11508.1 14 -5.7 89.95 10 SrO 0.05 1200 1 1 17 -4.3

z a 80.00 10 SrO 10 1400 11 18 4.5 99.0 0.5 SrO 0.5 810 12 20 -3.3 98.50.5 C0D 0.5 850 27 -3.5

MnO 0.5 98.5 0.5 C60 05 1700 40 -4.2

515 0 0.5 98.5 0.5 COO 0.5 1000 22 35 -4.5

810 0.5 $10 72 98.5 0.5 MnO 0.5 sb o 20 1800 40 50 -4.7

Sb O 0.5

zQl 98.5 0.5 MnO 0.5 1300 32 40 3.8

BaO 0.5 98.5 0.5 MnO 0.5 1250 30 40 -3.8

SrO 0.5 98.5 0.5 sb o 0.5 1300 20 30 -4.7

$10 0.5 98.5 0.5 BaO 0.5 750 17 25 -7.0

SrO 0.5 C0D 0.5 98.0 0.5 MnO 0.5 1800 40 50 -1.5

$6 0., 0.5 C00 0.5 98.0 0.5 MnO 0.5 800 29- 35 -2.5

BaO 0.5 CO0 0.5 98.0 0.5 MnO 0.5 770 26 35 -3.0 SrO 0.5 COO 0.5 98.0 0.5515 0 0.5 1500 33 40 -2.7

BaO 0.5 COO 0.5 98.0 0.5 S12 0 0.5 Sio (72 1450 31 35 -22 SrO 0.5 COO0.5 815 0 20 98.0 0.5 13:10 0.5 B1 0 8) 880 18 25 -3.3

SrO 0.5 MnO 0.5 98.0 0.5 Sb 0 0.5 1650 35 40 -3.1

BaO 0.5 I MnO 0.5 98.0 0.5 813 0;, 0.5 1600 33 40 -24 SrO 0.5 MnO 0.598.0 0.5 8110 0.5 1000 21 35 -2.5

SrO 0.5 Sb O 0.5 98 0 0 5 BaO 0.5 1050 18 30 -3.0

SrO 0.5

EXAMPLE '4- The fabrication process and testing method were the same asthose of Example 2 "and the thickness of the high resistance layer wasvaried with the results as EXAMPLE 5 1 Starting materials of Table 5were fabricated into voltage dependent resistors by the same process asin Example 1 The pressed bodies were sintered at a temshown in Table 4.It is easily Understood that the "ability 5 emt r'e between 1,000C to1,450C for 5 hours after to withstand impulses increases w1th an mcrease1n the covering h id surface i h coating pastes as li thlckness of thchlgh reslstance layer and the t r of in Table 5. The test conditionswere the same as those Change of the Caused y the bollmg test of Example2. The electric characteristics of resulting creases with anincrease ofthickness of the high I'CSIS- 1O resistors are Shown i T bl 5 tancelayer.

Size of disc: 32 mm in dia. and 20 mm in thickness Size of disc: 32mm'india. and 20 mm. Sinterin g: l,200C for 5 hours Thickness of highresistive layer; 30 1..

Table 4 Composition Solid ingredient Thickness Electric Characteristicsof of Sintered of Paste of High- Resultant Resistor Body (mol ResistiveC(V) n Impulse Boiling (mol layer Withstand Test (a) (at lmA) 0.1-1 mA(KA) AC 10 600 16 30 -4.3 {Sio 90 30 605 16 35 t -2.9 Bi O 10 100 605 1640 -3.2 300 615 16 50 -1.2 10 600 14 30 -4.2 ZnO (99.0) {513 0 (90 30603 15 35 -2.7 136 0, (0.5 13i 0 10 100 605 15 40 -2.2 300 610 15 45 47C00 (0.5 10 590 15 -4.8 1:1 0 90 600 15 -3.1 {Bi 0 10) 100 605 15 -3.3300 610' 16 -2.7 10 605 17 35 -3.3 $10 (72 30 620 17 45 -1.7 Sb O (20100 620 17 -1.2 131 0 8) 300 630 18 60 -1.0 10 200 43 ,30 -2.1 {SiO (9030 2040 45 40 1.5 131 0 10 100 2070 45 45 -1.1 v 300 2100 46 45 -0.5 101950 43 30 -3.3 2X10 97.5 86 0 (90 30 2010 46 40 25 mp3 (0.5 {B1 0 10100 2030 46 40 20 C00 (0.5 300 2050 46 50 -1.6 10 2000 .44 30 -4.7 MnO0.5 ln Q, (90 30 2015 r 46 40 -3.1 S6 0 1.0 {Bi o (10) 100 2050 46 -2.2300 2100 47 -1.8 10 2050 46 50 -1.2 sio 72 30 2100 48 60 -0.5 51: 0,, 20100 2120 50 -0.5 13i c (8) 300 2150 50 -0.4

Table 5 Composition Solid Ingredient Sintering Electric Characteristicsof of Sintered of Paste Temp. Resultant Resistor Body (mol. (C) C (V) nImpulse Boiling (mol. (at 1 mA) 0.1-1 mA Withstand Test AC(%) 1000 12001 1 15 -9.5 sio (50 1100 850 14 17 -7.2 1200 605 15 20 -5.0 131 0 (50)1300 420 13 18 -5.1 1450 280 11 18 -5.3 1000 1220 13 1 20 "-7.7 sio 1100870 14 25 -4.1 1200 605 16 35 29 131,0 10) 1300 450 16 35 -2.9 1450 30014 30 -3.5 1000 1250 12" 20 -7.0 Sio 1100,. 900 14 25 -5.1 1200 615 v 1530 -3.5 Bi O 0 1300. 470 14 23 3.7 1450 330 14 20 -4.0 ZnO (99.0 Sb 0 301000 1200 11 15 -8.1 I 1200 600 14 20 -5.3 Bi o (0.5 Bigot, (70) 1450300 13 18 57 C00 (05) 1000 1100v 13 28 -4.1 Sb,0 90 1200 603- 15 g 35-2.7 {1300 10 1450 285 14 1 30 -3.3 Sb,O 100 1000 1220 -12 20 5.0 1200610 '14" 25 -3.5 Bi Q, 0 1450 310 '13 20 -4.0 1:1 0.l (50) 1000 1200 12*u 15 -7.5 I 1200 595 '14 20 -5.7 1500., 50 1450 320 12 18 -6.0

Table Composition Solid Ingredient Sintering Electric Characteristics ofof Sintercd of Paste Temp. Resultant Resistor Body (mol. 7:) (C) C (V) nlmpulse Boiling (mol. 7:) (at 1 mA) 0.1-1 mA Withstand Test AC(%) 1110;, 90) 1000 1230 v 13 25 -4.7 1200 v 600 15 35 -3.1 Bi Q, 1450 295 -3.6In Q, 100) 1000 1200 14 25 -5.1 l 1200 610 14 -3.5 131 0 0) 1450 305 1430 -4.0 1000 1250 14 -3.6 $10 72 1100 910 15 -2.1 {515 0; 20 1200 620 17-1.7 Bi Q, (8) 1300 430 16 40 -l.8 1450 300 15 40 -2.3 SiO (90) 10003800 38 30 2.9 1200 2040 45 40 -1.5 Bi Q, 10) M 1200 42 35 -2.0 10003900 41 35 -3.5 ZnO (97.5) {sb o (90) 1200 2010 46 40 -2.5 B1 0 (0.5) BiO (10) 1450 1250 43 35 -2.7 C00 (0.5) 1000 4000 42 35. 4.7 MnO (0.5){In- 0;, (90) 1200 2015 46 40 -3.1 Sb Q, (1.0) Bi O (10) 1450 1300 40 403.5 1000 4050 40 40 -1.3 SiO (72) 1 100 3200 44 550 0.9 {Sb O (20) 12002100 48 -0.5 Bi O 8) 1300 1550 1 44 5O -l.l 1450 1300 40 45 -1.5

EXAMPLE 6 resistors are shown in Table 6. The excellent ability to Th9 irC of Table 6 were. pressed and covered by withstand impulses and thesmall change with C-value coating paste comprising the same oxldesas theaddl- 30 were obtained by coating paste containing the same tives in thebody. The bodies. were sintered in air for 5 materials as the additivesin the sintered body. hours. The test conditions were the same as thoseof Size of disc; 32 mm in dia. and 20 mm in thickness Example 2. Theelectric characteristics of the resultant Thickness of high resistivelayer; 30p.

Table 6 Composition of Solid Ingredients Sintering ElectricCharacteristics of Sintered body of Paste Temp. Resultant Resistor (mol.I C(V) n Impulse Boiling ZnO Bi Q, Further I (mol. (C) (at lmA)Withstand Test Additives 0.1-lmA (KA) AC (7c) 99.5 0.5 131 0 100 12004000 4.1 10 -7.5 99.5 CoO 0.5 C00 100 1200 2200 3.9 10 -6.2 99.5 MnO 0.5MnO 100 1200 2600 3.4 10 -5.3 99.5 Sb O 0.5 Sb O 100 1200 3000 3.7 12-62 99.5 BaO 0.5 B210 100 I200 1600 I5 -7.0 99.5 s10 0.5 SrO 1200 15007.8 12 -s.3 99.5 U0 0.5 U0 100 1200 2000 4.1 10 -7.9 99.5 PbO 0.5 PbO100 1200 4000 4.3 12 -7.1 99.0 0.5 C00 0.5 Bi O;,(5O) C00 (50) 1200 600I5 22 -3.5 99.0 0.5 MnO 0.5 81 0450 MnO(50) 1200 1000 23 25 -3.7 99.00.5 Sb O- 0.5 Bi O (5O) $0 0.50 1200 985 8.3 18 -4.2 99.0 0.5 13:10 0.5131 0450 880(50) 1200 820 1 1 20 -3 3 99.0 0.5 SrO 0.5 Bi O (50) Sro 501200 800 12 20 37 S 0.5 SrO 5O MnO 0.5 MnO 50 99.0 1300 3500 30 35 -4.7

BaO 0.5 Bao I 50 BaO 0.5 8:30 50 I 99.0 1 100 2,000 20 30 -3.3

SrO 0.5 SrO 50 C00 0.5 131.0 50 98.0 1.0 C00 25 1200 1800 15 25 -2.7

MnO 0.5 MnO 23 B210 0.5 Bi O 50 98.0 1.0 B210 25 1200 1650 14 20 -3.5

SrO 0.5 SrO 25 C00 0.5 131 0., 20 97.5 0.5 MnO 0.5 C00 20 1200 2000 4655 -l .7

MnO $15 0.. 1 .0 $15 0., 60 C00 0.5 131 0. 10 97.0 0.5 MnO 0.5 C00 101200 2600 50 60 -0.5

$15 0. 1.0 MnO 10 Sh O,, 40 5110 0.5 $110 30 C00 0.5 131 0. 10

Table 6 Composition of Solid Ingredients Sintering ElectricCharacteristics of Sintered body of Paste Temp Resultant Resistor (mol.C(V) n Impulse Boiling ZnO Bi O Further (moi. (C) (at lmA) WithstandTest Additives 0.llmA (KA) AC 76) 97.0 0.5 MnO 0.5 MnO 1200 2800 50 60-05 sb o 1 .0 Sb O 60 Cr O 0.5 CF 10 C0D 0.5 lsi o 1o MnO 0.5 CoO 5 96.50.5 Sh O;, 1.0 MnO 5 1200 4400 55 70 -().3

Cr O 0.5 sb o sio 0.5 Cr O 5 SrO 50 C0D 0.5 Bi o 5 MnO 0.5 CoO 5 94.00.5 Sb O 1.0 MnO 5 513 0,, 20 1200 5600 60 70 -0.3 Cr O 0.5 Cr O: 3 SiO;2.0 SiO 60 NiO l.0 NK) 2 COO 0.5 mp, 25 1000 3800 35 -l .2 98.0 0.5 MnO05 CO0 25 1200 1800 50 -0.8

MnO 25 Sb O 0.5 Sb O 25 1450 850 41 40 -l .3

2. A voltage dependent resistor according to claim 1, in which said zincoxide sintered body consists essentially of, as a major part, 99.9 to80.0 mole percent of zinc oxide, and, as an additive, 0.05 to 10.0 molepercent of bismuth oxide (Bi O and 0.05 to 10.0 'mole percent, in total,of at least one member selected from the group consisting of cobaltoxide (CoO), manganese oxide (MnO), antimony oxide (Sb- 0 barium oxide(BaO), strontium oxide (SrO) and lead oxide (PbO).

1. A VOLTAGE DEPENDENT RESISTOR COMPRISING A ZINC OXIDE SINTERED BODYITSELF HAVING VOLTAGE DEPENDENT PROPERITIES, A COVERING LAYER ON THESIDE SURFACE OF SAID BODY HAVING A THICKNESS OF MORE THAN 10U THEPORTION OF SAID COVERING LAYER UP TO A DEPTH OF 3U FROM THE SIDE SURFACEBEING COMPRISED OF
 2. A voltage dependent resistor according to claim 1,in which said zinc oxide sintered body consists essentially of, as amajor part, 99.9 to 80.0 mole percent of zinc oxide, and, as anadditive, 0.05 to 10.0 mole percent of bismuth oxide (Bi2O3) and 0.05 to10.0 mole percent, in total, of at least one member selected from thegroup consisting of cobalt oxide (CoO), manganese oxide (MnO), antimonyoxide (Sb2O3), barium oxide (BaO), strontium oxide (SrO) and lead oxide(PbO).